Charge transfer devices (CTD's) are well known in the art. Such a device comprises a semiconductor substrate, typically with a single surface channel, to which an arrangement of electrodes is coupled for defining a path. The electrodes are electrically isolated from one another and adapted to move charge packets along a sequence of potential wells formed in response to fixed amplitude control phase-related voltages applied to the electrodes in sets in a now well-understood manner.
A variety of input structures are known for introducing charge into such devices. Each of these input structures responds to an input information bearing and so varying signal voltage which is determined with respect to some reference potential. As a consequence, the charge which is developed in the input structure is a function of the applied signal.
A prior art improvement over a single-channel device uses two channels where the output information signal is taken as the difference between the outputs of the two channels. By using such a differential mode, the effects of dark current, temperature dependent drift, clock pickup, and even harmonic distortions detrimental in single-channel devices, are significantly reduced since they appear as common mode noise voltages and are thus cancelled by a difference amplifier employed at the output.
To realize the full benefit of the two-channel approach, the charge has to be introduced in an exactly complementary form into the two transfer channels. In prior art devices using two channels, two separate signal information inputs were provided to generate two charge packets independently where the charges had no fixed relationship with one another. Further, the generation of the two charge packets required amplifier circuits with attending losses in linearity and symmetry as is well known.